Electric furnace



Dec. l5, 1931'. Ry c. BENNER ET AL ELECTRIC FURNACE Filed Dec. '7,

1928 4 Sheets-Sheet l zl n / CARBON DUST lNvNToRS R mond, C. Bannv Bf ffff@ ATTORNEY j Dec. 15, 1931. R c. BENNER ET AL 1,837,178

ELECTRIC FURNACE Filed Dec. 7, 1928 4 Sheets-Sheet 2 C ARBON DU ST .iii/0f WATER COO LER l 20 WAT ER GOOLER INVENTORS Ra und C. Bmnw v e, T. Easter Dec. l5, 1931. R. c. BENNER ET AL ELECTRIC FURNACE Filed Dec. 7, 1928 4 Sheets-Sheet 5 S an DIN z T Y.R m nem la MRaGCMA ffm 1 RAYMOND GBENR 1N V E NTO RS axioms Mmm GLARLNCE i ,HRW KE ATTORNEY Dec. 15, 1931. R. c. BENNER ET A1.

ELECTRIC FURNACE Filed Deo. 7, 1928 4 Sheets-Sheet 4 Patented Dec. 15, 1931 UNITED STATES PATENT OFFICE RAYMOND C. BENNER AND GEORGE J. EASTER, F NIAGARA FALLS, NEW YORK, CLARENCE E. HAWKE, 0F MET'UCHEN, NEW JERSEY, ASSIGNORS TOTHIE CABBORUN- DUH COMPANY, 0F NIAGARA `FALLS, NEW YORK, A CORPORATION` 0F PENNSYL- VANLA ELEcrnIc Fnmucn Application led December 7, 1998. Serial No. 324,416.

Our invention relates to the prolongation of the useful life of non-metallic resistors such as are employed in electric furnaces for temperatures up to 1400" C. A related object of our invention is to raise the temperature limit at which such resistors can be profitably used in a commercial way up to 1600 C.-1800 C.

Our invention applies particularly to rigid lo resistors of silicon carbide. Such resistors possess much higher specific resistance than carbon, graphite or metals and may, accordingly, be utilized in a form which is much more convenient to handle. Y

From a purely thermal standpoint such resistors can be .used at very high temperatures, silicon carbide for instance can be subjected to a temperature of 2200D C. before decomposing. It has been found however, that at '20 temperatures above 1100o C. the resistors have slowly increased in resistance due to the oxidation of the carbide byy gases in contact therewith. In the case of some silicon carbide rods the resistance doubles in the course of 1000 hours of use due to this phenomcnon. The operating temperatures for such rods for any reasonable life is ordinarily found to be limited to about 11100o C.

The useful life of such resistors also depends greatly on the avoidance of arcing at the point of contact between the non-metal- 'lic resistor and the metallic conductors supplying current thereto. This tendency to arcing and the seriousness of its effects are materially increased as the temperature of the contact is increased so that it is necessary to adopt means for keeping this contact as cool as possible, particularly where as, in our invention, the temperature of the resistors is raised above 1400 C. It has accordingly been necessary to provide means for cooling the ends, which will be hereinafter described.

We have found that these resistors can be protected from oxidation in such a manner as to greatly increase their useful life at the temperatures heretofore used and that the possible operating temperature can be increased as noted above by surrounding the heated portion of the resistor with a gas, for

example producer gas, nitrogen or carbon monoxide, which is inca able of reacting with it to oxidize the car ide. Such a gas may be introduced as such in the vicinity of f through the walls of the compartment conn taining material to be heated and around the resistors which in such furnaces are wound upon said walls. It is also known that for '4 limited service such metallic windings have been protected by embedding them in powdered charcoal, which is sometimes mixed with powdered oxides such as alumina or magnesia; and that graphite resistors have also been embedded in lampblack or similar materials to increase their life. We have no nowledge however of any utilization of similar means for protection of carbide resistors despite the fact that the latter have been in common use and the difliculties at,

tendant upon their oxidation well known for a number of years.

The various means which we use for protecting the resistor rods and increasing the temperature at which they can be used are illustrated in the accompanying drawings in which:

Figure 1 isa vertical section of a furnace 1n a plane parallel to a resistor rod protected in accordance with our invention and showing one of the resistor rods and terminal cooling means for the same;

Figure 2 is a vertical section through the center of the furnace taken on the line 2-2 of Figure 1;

Figure 3 is a perspective view of 'a metal radiator plate;

Figure is an axial section of .aresistoigyy rod surrounded byal'modified form 'of en-l closure which may be described as a cartridge,

type;

eating, a .f1-meent; terminals;

Figure 8 is a view similar to Figure 1 illustrating a modification in which 'resistor plates are used under adjustable pressure; ,A

Figure 9 is a vertical section of a modified furnace inwhich: ,heY interior ofthe mule n is kept subjectedftareducin `atmosphere,

the V `resistors being '-embedde Lin the vside c Figure 10 is-.a vertical lsection of another form ojflfurnace in-,whichthemuflle .is subhere, the resistors eing directly in. the vmu e; l

Figure 11,is a .vertical section of a modified furnacezin whichreducing gas' is introduced directly into the heating chamber; and

Figure 12 is a vertica Figure 11. I,

hroughout .the drawings 1=' designates rigid resistor rods consisting mainly of some form ofv` carbide, such as silicon carbide. In Figures 1, 2,- 6 7 :and9 these rods are surrounded forla Vgreater' part of their length by granular carbon-or carbon dust 2. This carbon may be made `for example of crushed petroleum coke which may act as a mechanical support for the rod'as well as protecting it from oxidation as previously explained. Adjacent the resistor rods is placed muiiie 4'whose walls 5v are made of a suitable refractory material having a thermal conductivity in excess of 0.006 calories /cm.3 /sec. C. Fused alumina is an example of such a refractory, although we prefer to use re'- crystallized siliconcarbide, such as is described in U. S. patent to F. J. Tone, No. 709,808, or silicon carbide bonded in any of the manners well known in the manufacture of refractories, bonds containing halides of the alkaline earthsbeing particularly desirable onaccount of their stability in re-- ducing atmospheres as describedin cepending application Serial N o. 269,075, filed April 11, 1928, 'by Benner and Baumann. It is further desirable that the walls be protected with a glaze of a reduction resistant type such as one composed of alumina and an alkaline earth halide as describedby Benner and Baumanns copending application Serial No. 228,492 filed October 24, 1927, in order to continuously protect them against oxidation and also to minimize the penetration oi' air into the space adjacent the resistor. The

section through the1 center of thefurnace taken on Yline A-A of#` muiile is shown in the drawings as surrounded by granular carbon or carbon dust, although this is not essential to our invention. The supports for the muiile and outer wall 3 of the furnace are made of any suitable refractory'material, preferably' algvood ther- I-malinsulator, such for example as high grade .calcined kaolin or diatomaceous earth.

Spring -contacts ,designatedV generally las '19" are shown in Figures l, 7 and 8. lThese take care lOrf change of length of the resistor 'and contact due to thermal expansionand maintain Van even pressure of the Contact against the'resistor at all times.

Figure 1 shows a furnace equipped with an improved type of air cooler for the contacting terminals for the purpose of preventing arcing as previously described. These terminals consist of a tip 6 hollowed to receive the end of the resistor and composed vof chrome iron (approximately 27% chromium) or other high heat resisting metal welded to a portion 7 composed of a cheaper metal of greater thermal electrical conductivity to which eiectricity is supplied through a suitable lead not shown. The portion 7 is surrounded by. a corrugated metal radiator 8 in as close contact as possible therewith. Other suitable types of radiator for `this pur-. pose are the plate form shown in Figure 3 or the. axial fin type shown in Figures 4 and 5. The inner end of this radiatorv vmayserve to retain thecarbomwithin the furnace, although it is desirable that the dimensions of the carbon particles and theclearance of the terminal where it passes through the outer wall of the furnace be such that the carbon can not become wedged betweenthe terminal and the wall as it is essential that theterminal move freely to permit it to maintain perfect contact with the resistor. In furnaces having a number of rods there may be individual metal radiators for each rod, or a single radiator may be in contact with the terminals of several rods. Cooling may be increasedby directing a blast of air againstfthe radiator if desired.

Figure 7 is similar to Figure 1 except that the cooling of the contacts is accomplished by means of a water cooler thus permitting the furnace to be operated at materially higher temperatures than is possible with the furnace shown in Figure 1. The water cooler in F lgure 7 is made up of a portion 65L of chrome iron y(approximately 27% chromium) or other high heatv resisting metal welded to a better conducting portion 7a which is direct- -ly cooled bywa'ter flowing therethrough and through which electrical contact is made by lead-in wires 20. iVhile both air cooling and water cooling means are illustrated' alone, these two methods may be combined by placing water coolers at the outer ends of 7 shown in Figure 1.

A special form of our invention is shown in Figure 6 whereby the resistor, together with the carbon surrounding it, is assembled in cartridge form making a unit which can be inserted or removed from a furnace with a minimum amount of inconvenience and loss of time. In this form the resistor 1 surrounded by carbon 2 is carried Within a shell 5 of silicon carbide, either recrystallized or suitably bonded as above described. Contact with the resistor is made through suitable conducting plugs 24 of the same material as the resistor or of graphite inserted in the ends of the outer shell and separated therefrom by a layer of insulating material 25. One of these plugs may be fastened securely in lace, but at least one must be free to yie d slightly to compensate for differences in expansion of the resistor and the outer shell upon heating. An additional advantage of this type of construction is that the end plugs have alarger cross section than the resistor rod causing less localized heating near the metal contacts.

In the modification shown in Figure 8, resistor plates 10 of silicon carbide, which are subjected to adjustable pressure in order to permit variation of the over-all resistance, are used to replace the resistor rods in Figure 7. The silicon carbide plates may be interspersed with graphite plates at suitable points in order to reduce the total resistance and Vary the heat distribution as may seem desirable. Carbon dust is used as before to embed the resistor.

In the form'of furnace shown in Figure 9 the side walls 11 of the heating chamber are made of porous recrystallized silicon carbide so that in addition to protecting the resistor rods the atmos here of carbon monoxide passes 'through t e walls 11 into the heating xber 12 surrounding the resistor rods to assist the production of carbon monoxide from the carbon grains and its diffusion throu h the walls 11. The supply of carbon may e replenished as required through the openings 17 which are ordinarily closed by the doors 18.

In the form of furnace shown in Figure 10 theresistors l are placed adjacent a porous wall 13 of recrystallized silicon carbide which conducts the heat to granular carbon contained Within a chamber 14 Within the Wall of the furnace. This carbon is also shown extending down through the side Walls of the furnace where it is separated from the interior of the heating chamber by Wall 15 similar to the wall 13. lith this construction carbon monoxide is formed and diffuses through the porous Walls to the interior of the muflie affording protection to the res- 1 sistors.- Fresh carbon may be added through .plu

an opening which is ordinarily closed by the 16, or through the openings 1711s descri ed in connection with Figure 9. An air pressure may also be maintained in the compartment which is vfilled with carbonas described in connection with Figure 9. .v l f Figures 11 and 12'show the directuse of a non-reacting gas independent of granular carbon to afford protection for the resistors. Producer gas or other suitable gas is introduced into the muiiie through-conduits -22 "Y preferably inthe vicinity of the ports Where the resistors enter the furnacewall: Theow of this gas may be controlled bymeans yof valves 23 so that the desired atmosphere will S0 be maintained within the -heatingchamber without Waste of gas. lThis type offu'r'nac'e is particularly well adapted for heat treating Work in which the metal must protected from scaling. Y f 85 A continuation in part of'th'eprese'nt a plication, U. S. Serial No. 471,714, filed'J u y 30, 1930, claims specifically a furnace in which resistor elements are mounted inside the heating chamber. A

'Weclairm f f 1. In an electrical resistance furnace a silicon carbide resistor, an adjacent mullie whose walls are composedof ysilicon carbide, and means for maintaining anon-oxidizing atmosphere around 'the resistorl and muille. Y

2. In an electrical resistance'l furnace rigid silicon carbide resistors, a muflle having walls of recrystallized silicon carbide," and permeable mass of granular carbon adjacent said lr`esistors, whereby fa' reducing 'atniospheregdifl fuses through the Wall into the mufileto protectwareinthemufile. f

3. In an electrical resistance furnace` a plu# rality of silicon carbide resistor elements mounted in series, means for subjecting said elements to adjustable pressure for varying the resistance and means for protecting said elements from oxidation.

4. In an electricfurnace, aheating chamber for ware to be treated, a compartment containing granular carbon adjacent said heating chamber, silicon carbide resistor elements in said compartment, and a Wall of conducting refractory separating said chamber and compartment, said conducting wall being permeable to gases, whereby gases produced by the oxidation of the granular carbon may pass into the heating. chamber. I

5. In an electric furnace, a heating chamber for Ware to be treated, a compartment containing granular carbon adjacent said heating chamber, silicon carbide resistor elements in said compartment, a wall of conducting refractory separating said chamber and compartment, said conducting wall being permeable to gases, and means-for maintaining gas pressure within said compartment, Whereby gas from the compartment passes into said heating chamber.

6. In an electric furnace, the combination. of a silicon carbide heating resistor, means for preventing oxidation thereof, means for coolmg the contacts through which electriclty 5 is supplied to said resistor, and a heating chamber adjacent said resistor, said chamber having a Wall of refractory material possessing athermal conductivity in excess of 0.006 calories/cm,3/sec./ C. c 7. -A resistor unit for electric furnaces comprising a heating resistor embedded in granu-` lar carbon within anannular casing of refractory material of high heat conductivity, endj plugs forming electrical contact with the :resistorbut insulated from the annular casingysaid end plugs also servingto retain the lcarbon in position around the heating resistor.

8. An electric furnace containing silicon Y carbide resistors, cooled terminals for said resistors,a compartment having silicon carbide walls, and powdered carbon around said resistors and between said resistors and said Walls, whereby the 'resistors and at least a portion of the walls are protected from oxidation during the operation of the furnace.

9. The electric furnace described in claim 8 in which the walls of the vcompartment are porous whereby reducing gases yfrom the heated carbon dust may pass through the walls to protect also the ware beingy heated in the furnace.

10. In anvelectric furnace, a heating chamber, a silicon carbide ,'resistor in close Vproximity to the outer wall of said chamber, granular carbon surrounding said resistor, the amount of carbon between the resistor and the outer portions of the furnace being large in comparison with the amount inter- 4o posed between the resistor and the heating chamber.

RAYMOND C. BENNER. GEORGE J. EASTER. CLARENCE E. H AWKE. 

